Web Development

Candidate Keys in DBMS for Database Integrity & Consistency

Published: 1st September, 2023

Vibha Gupta

Technical Content Writer at almaBetter

A candidate key is a set of one or more columns that can uniquely identify a row within a table. It is also known as a "primary key." Read further for more!

In database management systems (DBMS), candidate keys play a crucial role in maintaining the integrity and consistency of a database. A candidate key in DBMS is a set of one or more columns that can uniquely identify a row within a table. It is also known as a "unique identifier" or "primary key." In this article, we will define candidate keys in DBMS (database management system), and discuss their significance and how they are used to ensure the accuracy and reliability of a database.

What is Candidate Key in DBMS?

What is a candidate key in DBMS? - A candidate key is a set of one or more columns that can be used to uniquely identify a row within a table. It ensures that no two rows within a table can have the same values for the columns that make up a candidate key. While a table can have multiple candidate keys, only one can be chosen as the primary key, which is the official unique identifier for a row within the table.

For instance, let's consider a table called "Employees" that contains information about the employees of a company. This table has columns such as EmployeeID, FirstName, LastName, and Email. In this case, the column "EmployeeID" could be a candidate key as it is unique for each employee and can be used to identify a specific row within the table.

Creating a Candidate Key in DBMS with Example

Creating a candidate key example is a straightforward process. In most DBMS, you can define a primary key constraint on one or more columns of a table to create a candidate key in DBMS. The syntax for creating a primary key constraint may vary depending on the specific DBMS you are using. However, the general structure is as follows:

CREATE TABLE Employees (
   FirstName VARCHAR(255NOT NULL,
   LastName VARCHAR(255NOT NULL,

In the example of candidate key above, the primary key constraint is placed on the "EmployeeID" column, making it a candidate key for the "Employees" table. This ensures that no two rows have the same value for the "EmployeeID" column.

Types of Candidate Keys

Candidate keys can be categorized into various types based on their composition and uniqueness. Understanding these types can help in choosing the most appropriate candidate key for a given scenario. The different types of candidate keys include:

1. Simple Candidate Key

A simple candidate key is a primary key that consists of a single column. For example, the "EmployeeID" column mentioned earlier can be considered a simple candidate key in DBMS.

2. Composite Candidate Key

A composite candidate key is a primary key that comprises multiple columns. For instance, a table of "Orders" might have a composite primary key made up of the columns "OrderID" and "CustomerID." These columns uniquely identify a specific order placed by a customer.

3. Super Key

A super key is a set of one or more columns that can identify a row uniquely, although it may not be minimal. A super key can be either simple or composite.

4. Minimal Super Key

A minimal super key is a super key with the least number of attributes required to identify a row uniquely.

Importance of Candidate Keys

You must be wondering how to find candidate keys in DBMS? Candidate keys are of utmost importance in maintaining the integrity and consistency of a database. They ensure that each row within a table is unique and can be identified independently. This characteristic allows for accurate data retrieval and helps prevent data duplication and errors.

Candidate keys also play a vital role in enforcing relationships between tables in a database. Using foreign keys that reference the primary key of another table, the relationship between the two tables is established, ensuring consistent and accurate data.

For example, let's consider the "Employees" table mentioned earlier. If there is a separate "Department" table, having a primary key column named "DepartmentID," a foreign key can be created in the "Employees" table that references the "DepartmentID" column. This linkage ensures that each employee is associated with their department, maintaining data consistency.

Additionally, candidate keys contribute to the performance of a database by enabling faster and more efficient data retrieval. The DBMS can utilize the primary key to quickly locate specific rows within a table without searching the entire table. This becomes particularly significant in large databases where performance is a critical concern.

Secondary Keys and Non-Key Attributes

Apart from candidate keys, a table may also have secondary keys, which are unique but not chosen as the primary key. Secondary keys can improve data retrieval and provide an alternate means of identifying a row.

On the other hand, non-key attributes are columns within a table that do not form part of any key and do not provide unique identification for a row. Instead, they offer additional information about the entity represented by the row. For example, in our "Employees" table, the "FirstName" and "LastName" columns are non-key attributes.

Candidate Key vs. Primary Key

It is important to note that while "candidate key" and "primary key" are often used interchangeably, they do not have the same meaning. A candidate key is a set of one or more columns that can uniquely identify a row within a table, while a primary key is the chosen candidate key in DBMS that serves as the official unique identifier for a row. A table can have multiple candidate keys but only one primary key.

Surrogate Key

A surrogate key is a specific type of primary key used as a unique identifier for a row rather than relying on a natural key. A natural key is a column or set of columns that inherently hold meaning within the data, such as the "EmployeeID" column in our "Employees" table example. In contrast, a surrogate key is a unique identifier generated by the database with no inherent meaning within the data.

The primary advantage of using a surrogate key is its efficiency as a primary key. Surrogate keys are generally smaller and more efficient to use, and they can be changed without affecting the data. This flexibility may not be possible with a natural key.

One typical example of a surrogate key is an auto-incrementing integer. These keys are often used as primary keys and are assigned by the database system, incrementing by a fixed number and guaranteeing uniqueness.

Choosing the Right Candidate Key in DBMS

Selecting the appropriate candidate key is crucial during database design. The choice of candidate key can significantly impact the database’s performance, scalability, and flexibility of the database. Several factors should be considered when choosing a candidate key:


The key should be unique and not subject to change.

Minimum Size

The key should be as small as possible to optimize performance.


The key should remain stable over time to maintain data consistency.

Business Meaning

The key should have business meaning and be relevant to the data in the table.

By taking these factors into account, it becomes easier to choose the right candidate key that meets the requirements of the database and ensures its proper functioning in the long run.


We have learned the candidate key definition, and ways of finding candidate key in DBMS. Candidate keys are vital components in DBMS that help maintain the integrity and consistency of databases. They ensure each row within a table is unique and can be identified independently. By enforcing relationships between tables, candidate keys contribute to data consistency. Moreover, they enhance the performance of a database by facilitating faster and more efficient data retrieval.

Understanding the various types of candidate keys, such as simple and composite candidate keys, super keys, and minimal super keys, is crucial for effective database management. Additionally, considering secondary keys and non-key attributes further enhances the versatility and usability of a database.

Database designers can establish the most appropriate unique identifier for their tables by differentiating between candidate and primary keys. Surrogate keys offer advantages such as efficiency and flexibility, making them attractive in many scenarios.

Choosing the right candidate key is a critical aspect of database design. Considering factors like uniqueness, size, stability, and business meaning helps ensure the optimal functioning of the database and its long-term effectiveness.

In conclusion, candidate keys are the foundation of a well-designed database and database approach, ensuring data integrity, consistency, and performance. By choosing the right candidate keys, database administrators can create robust and efficient databases that meet the needs of their organizations.

Related Articles

Top Tutorials

Made with heartin Bengaluru, India
  • Official Address
  • 4th floor, 133/2, Janardhan Towers, Residency Road, Bengaluru, Karnataka, 560025
  • Communication Address
  • 4th floor, 315 Work Avenue, Siddhivinayak Tower, 152, 1st Cross Rd., 1st Block, Koramangala, Bengaluru, Karnataka, 560034
  • Follow Us
  • facebookinstagramlinkedintwitteryoutubetelegram

© 2024 AlmaBetter